The present invention relates to a niobium powder for a capacitor free of reduction in the capacitance, a sintered body using the powder and a capacitor using the sintered body.
There is a demand for capacitors for use in electronic instruments such as potable telephone and personal computer to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferably used because of its large capacitance per unit volume and good performance. In a tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body increased in surface area by pulverizing the tantalum powder. The former method of increasing the mass of the sintered body necessarily incurs enlargement of the capacitor shape and cannot satisfy the requirement for downsizing.
On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body is reduced or closed pores increase at the stage of sintering. Therefore, impregnation of the cathode agent in the later process becomes difficult.
As means for solving these problems, a capacitor using a sintered body of a material having a dielectric constant larger than that of tantalum is being studied. The material having a larger dielectric constant includes niobium. JP-A-55-157226 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method for producing a sintered element for capacitors, where an alloy mainly comprising tantalum, titanium, niobium, aluminum or the like is used as a valve-acting powder and the agglomerated powder of the alloy is molded under pressure into a niobium fine powder having a particle size of 2.0 xcexcm or less. The fine powder is sintered, the molded and sintered body is cut into fine pieces, a lead part is joined thereto, and then those pieces are again sintered. However, JP-A-55-157226 neither discloses nor suggests the tantalum content or the amount of niobium powder and moreover, the properties of the capacitor manufactured using this powder are not disclosed at all.
U.S. Pat. No. 4,084,965 discloses a capacitor manufactured using a niobium powder of 5.1 xcexcm obtained by hydrogenating a niobium ingot and pulverizing it. However, U.S. Pat. No. 4,084,965 neither discloses nor suggests the tantalum content and the amount of niobium powder.
Although niobium is deficient in the leakage current (hereinafter simply referred to as xe2x80x9cLC valuexe2x80x9d), the present inventors have previously proposed that the LC value can be improved by nitriding a part of niobium (see, JP-A-10-142004). Increased reduction in the LC value can be attained, for example, by elevating the sintering temperature at the time of manufacturing the above-described niobium sintered body. However, if the sintering temperature is elevated, there arises a problem that the product of the capacitance (simply xe2x80x9cCxe2x80x9d) per mass of the sintered body manufactured and the chemical forming voltage (simply xe2x80x9cVxe2x80x9d) at the time of forming a dielectric material on the surface of the sintered body (hereinafter the product is simply referred to as xe2x80x9cCV valuexe2x80x9d) becomes small.
The CV value is considered to be proportional to the surface area of the sintered body and, the surface area of the sintered body is estimated to depend on the specific surface area of the niobium powder with the same production conditions and molding conditions of niobium powder and with same sintering conditions in obtaining a sintered body. However, even if a niobium sintered body is manufactured from niobium powder using the same conditions in respective stages, the CV value of the manufactured niobium sintered body is not always the same but disadvantageously decreases.
By taking into account these problems, the present invention provides a niobium sintered body free of reduction in the CV value, a niobium powder for use in the manufacture of this niobium sintered body, and a capacitor using this niobium sintered body.
As a result of extensive investigations, the present inventors have developed a niobium powder for capacitors, which has a tantalum content (hereinafter xe2x80x9cppm by massxe2x80x9d is simply referred to as xe2x80x9cppmxe2x80x9d) reduced to a predetermined value or less thereby enabling the manufacture of a niobium sintered body free of reduction in the CV value. Based on this finding, the present invention has been accomplished. More specifically, the present invention includes the following embodiments.
(1) A niobium powder comprising niobium and tantalum, wherein the tantalum is present in an amount of at most about 700 ppm by mass.
(2) The niobium powder as described in 1 above, which is partially nitrided.
(3) The niobium powder as described in 2 above, wherein the amount nitrided is from about 10 to about 100,000 ppm by mass.
(4) A sintered body comprising the niobium powder described in 1 to 3 above.
(5) A method for producing a niobium sintered body, comprising sintering a niobium powder compact at a high temperature, wherein the niobium powder is the niobium powder described in any one of 1 to 3 above and heating the niobium powder under reduced pressure at about 500 to about 2,000xc2x0 C. for about 1 minute to about 10 hours.
(6) The method for producing a niobium sintered body as described in 5 above, wherein the niobium powder is obtained by granulating a niobium powder having an average primary particle size of about 1 xcexcm or less.
(7) A capacitor comprising a pair of electrodes having interposed therebetween a dielectric material, one of the electrodes being the niobium sintered body described in 4 above.
(8) The capacitor as described in 7 above, which has a dielectric material comprising niobium oxide formed by electrolytic oxidation.
(9) The capacitor as described in 7 above, wherein the other electrode is at least one material (compound) selected from the group consisting of an electrolytic solution, an organic semiconductor and an inorganic semiconductor.
(10) The capacitor as described in 7 above, wherein the other electrode is formed of at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer containing two or more repeating units represented by formula (1) or (2): 
wherein R1 to R4, which may be the same or different, each represents hydrogen, an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R5 is present only when X is a nitrogen atom and represents hydrogen or an alkyl group having from 1 to 6 carbon atoms, and R1 and R2, or R3 and R4 may be combined with each other to form a ring.
(11) The capacitor as described in 7 above, wherein the organic semiconductor is at least one selected from the group consisting of polypyrrole, polythiophene and substitution derivatives thereof.
(12) An electronic circuit using the capacitor as described in 7 above.
(13) Electronic equipment using the capacitor as described in 7 above.
One embodiment for obtaining the sintered body of the present invention is described below.
The raw material compound used for the niobium powder may be a generally available material. For example, a niobium powder obtained by the reduction of niobium halide with magnesium or sodium, by the sodium reduction of potassium fluoroniobate, by the molten-salt (NaCl+KCl) electrolysis of potassium fluoroniobate onto a nickel anode, or by the introduction of hydrogen into a metal niobium ingot and then pulverizing and dehydrogenating the ingot, may be used. The niobium powder obtained by these methods contains tantalum intermingled from the raw material.
The present inventors have found that it is important in the present invention to set the amount of tantalum contained in the niobium powder to about 700 ppm or less. If the tantalum content exceeds about 700 ppm, the VC value of the niobium sintered body manufactured may decrease. The amount of tantalum contained in the niobium powder can be reduced to about 700 ppm or less, for example, by purifying the manufactured niobium powder through electron beam dissolution or by mixing therewith a purified product.
In the present invention, the niobium powder suitably has an average primary particle size of about 1 xcexcm or less, preferably from about 1 to about 0.1 xcexcm. If the average primary particle size exceeds about 1 xcexcm, a sintered body characterized by high CV and low LC, which is one object of the present invention, can be hardly obtained and therefore, such a powder is not preferred as a raw material. For the average particle size of niobium powder as used in the present invention, a D50 value (a particle size where the cumulative % by mass is 50% by mass) measured using a particle size distribution measuring apparatus (xe2x80x9cMicrotrackxe2x80x9d, trade name) may be employed.
A niobium powder having an average primary particle size in the above-described range can be obtained, for example, by a method of pulverizing a sodium reductant of potassium fluoroniobate, a method of pulverizing and dehydrogenating a hydride of a niobium ingot or a method of producing niobium oxide through carbon reduction. When using the method of pulverizing and dehydrogenating a hydride of a niobium ingot is used, a niobium powder having a desired average particle size can be obtained by controlling the amount of the niobium ingot hydrogenated and the pulverization time in a pulverizer.
The niobium powder of the present invention is a niobium powder having the above-described tantalum content and is preferably a partially nitrided niobium powder. The amount nitrided is about 10 to about 100,000 ppm. In the case where after manufacturing a sintered body from the niobium powder, a dielectric material is formed on the surface of the sintered body, as described later and the leakage current (LC value) is measured in an aqueous phosphoric acid solution, the amount nitrided is preferably from about 500 to about 7,000 ppm so as to obtain a small LC value. The term xe2x80x9camount nitridedxe2x80x9d as used herein is not an amount of nitride adsorbed to the niobium powder but means an amount of niobium powder chemically nitrided.
The nitriding of the niobium powder can be performed by any one of liquid nitriding, ion nitriding and gas nitriding or by a combination thereof. A gas nitriding treatment making use of a nitrogen gas atmosphere is preferred because the apparatus is simple and the operation is easy. Gas nitriding using a nitrogen gas atmosphere can be achieved by allowing the above-described niobium powder to stand in a nitrogen atmosphere. A niobium powder nitrided in the desired amount can be obtained by a nitriding treatment at an atmosphere temperature of about 2,000xc2x0 C. or less for a standing time of about 1 minute to about 10 hours. The treatment time can be shortened by treating the niobium powder at a high temperature. The amount of the niobium powder nitrided can be controlled by selecting the conditions through a preliminary experiment for finding out a nitriding temperature and a nitriding time for a material to be nitrided.
The niobium powder of the present invention may be used after granulating the niobium powder into an appropriate shape or may be used by mixing an appropriate amount of a non-granulated niobium powder after the granulation. The granulation may be performed by a conventionally known method. Examples thereof include a method where a non-granulated niobium powder is allowed to stand at a high temperature in vacuum thereby becoming integrated (coagulation-solidified) and then cracked, and a method where an appropriate binder and a non-granulated niobium powder are mixed and the mixture is cracked.
At this time, the niobium powder and the binder may be kneaded using a solvent and after the kneading, the kneaded powder is dried and cracked. The binder is generally polyvinyl alcohol, acrylic resin or the like. For the solvent, one selected from acetone, alcohols, esters such as butyl acetate, and water can be used.
The thus-obtained niobium granulated product suitably has an average particle size of about 300 xcexcm or less, preferably about 200 xcexcm or less, more preferably from about 1 to about 200 xcexcm.
The niobium sintered body of the present invention is produced by sintering the above-described niobium powder. One example of the method for producing the sintered body is described below, however, the production method of the sintered body is not limited to this example. The niobium powder is pressure-molded into a predetermined shape and then heated under reduced pressure, for example, a pressure of 1.33xc3x9710xe2x88x924 Pa (Pascal), for a few minutes to a few hours at a temperature of about 500 to about 2,000xc2x0 C., preferably from about 900 to about 1,500xc2x0 C., more preferably from about 900 to about 1,250xc2x0 C., thereby obtaining the sintered body.
The lower limit of the temperature in sintering the niobium powder varies depending on the average particle size of the niobium powder and as the average particle size of the niobium powder is smaller, the lower limit of the temperature decreases. When the sintering temperature is varied while setting the average particle size to be constant, the sintered body manufactured suffers from a large LC value despite a large CV value and can hardly endure the practical use as a material of capacitors if the sintering temperature is low.
The production of a capacitor element is described below.
A lead wire comprising a valve-acting metal and having an appropriate shape and length is prepared and integrally molded at the pressure-molding of the above-described niobium powder so that a part of the lead wire is inserted into the inside of the compact where the lead wire can work out to a leading line of the sintered body.
By using the sintered body for one electrode and interposing a dielectric material between this electrode and another electrode, a capacitor element can be produced. The dielectric material of the capacitor is preferably a dielectric material comprising niobium oxide. The dielectric material comprising niobium oxide can be easily obtained by chemically forming the niobium sintered body used for one electrode in an electrolytic solution. The chemical forming of the niobium electrode in an electrolytic solution is usually performed using an aqueous protonic acid solution, for example, an aqueous solution of about 0.1% phosphoric acid or an aqueous sulfuric acid solution. In the case where the dielectric material comprising niobium oxide is obtained by chemically forming the niobium electrode in an electrolytic solution, the capacitor of the present invention is an electrolytic capacitor with the niobium side being the anode.
The other electrode of the capacitor of the present invention is not particularly limited, and for example, at least one material (compound) selected from electrolytic solutions, organic semiconductors and inorganic semiconductors well-known in the art of aluminum electrolytic capacitors may be used. Specific examples of the electrolytic solution include a dimethylformamide-ethylene glycol mixed solution having dissolved therein about 5% by mass of an isobutyltripropylammonium borotetrafluoride electrolyte, and a propylene carbonate-ethylene glycol mixed solution having dissolved therein about 7% by mass of tetraethylammonium borotetrafluoride. Specific examples of the organic semiconductor include an organic semiconductor comprising benzene-pyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer containing two or more repeating units represented by formula (1) or (2): 
wherein R1 to R4, which may be the same or different, each represents hydrogen, an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R5 is present only when X is a nitrogen atom and represents hydrogen or an alkyl group having from 1 to 6 carbon atoms, and R1 and R2, or R3 and R4 may be combined with each other to form a ring. For the dopant, any known dopant can be used without limit.
Specific examples of the inorganic semiconductor include an inorganic semiconductor mainly comprising lead dioxide or manganese dioxide, and an inorganic semiconductor comprising triiron tetraoxide. These semiconductors may be used individually or in combination of two or more thereof.
Examples of the polymer containing two or more repeating units represented by formula (1) or (2) include polyaniline, polyoxyethylene, polyphenylene sulfide, polythiophene, polyfuran, polypyrrole, polymethylpyrrole, and substitution derivatives and copolymers thereof. Among these, preferred are polypyrrole, polythiophene and substitution derivatives thereof (e.g., poly(3,4-ethylene-dioxothiophene).
When the organic or inorganic semiconductor is used and has an electrical conductivity of about 10xe2x88x922 to about 103 S.cmxe2x88x921, the manufactured capacitor can have a smaller impedance value and can be further increased in the capacitance at a high frequency.
When the other electrode is a solid, an electrical conductor layer may be provided thereon to attain good electrical contact with an exterior leading line (for example, lead frame).
The electrical conductor layer can be formed, for example, by the solidification of an electrically conducting paste, plating, metallization or formation of an electrically conducting resin film. Preferred examples of the electrically conducting paste include silver paste, copper paste, aluminum paste, carbonate paste and nickel paste, and these may be used individually or in combination of two or more thereof. In the case of using two or more kinds, the pastes may be mixed or individual pastes may be superposed one on another. The electrically conducting paste applied is then solidified by allowing it to stand in air or under heating. Examples of the plating include nickel plating, copper plating, silver plating and aluminum plating. Examples of the metal deposited include aluminum, nickel, copper and silver.
Specifically, for example, aluminum paste and silver paste are stacked in this order on the second electrode and the obtained laminate is sealed with epoxy resin or the like thereby constructing a capacitor. This capacitor may have a tantalum lead, which is sintering-molded integrally with the niobium sintered body or afterward welded.
The capacitor having the above-described construction of the present invention is applied with outer-jacketing, for example, jacketing by resin mold, resin case, metallic jacket case, dipping in resin or laminate film, and thereafter used as a capacitor product for various uses.
In the case where the other electrode is a liquid, the capacitor constructed by the above-described two electrodes and dielectric material is housed, for example, in a can electrically connected with the other electrode thereby forming a capacitor. In this case, the electrode side of the niobium sintered body is guided outside through a niobium or tantalum lead described above and at the same time, insulated from the can with an insulating rubber or the like.
By manufacturing a niobium sintered body as described above, a sintered body for capacitors free of reduction in the capacitance and a capacitor using the sintered body can be obtained.
The present invention is described in greater detail below by referring to the Examples, however, the present invention should not be construed as being limited to these Examples. Unless indicated otherwise herein, all parts, percents, ratios and the like are by weight.
The capacitor of the present invention is preferably used, for example, as a bypass capacitor, a coupling capacitor or a capacitor instead of a tantalum capacitor in an analog circuit or in a digital circuit.
A capacitor product, which has larger capacity per unit volume than that of tantalum, can be obtained by using the capacitor of the present invention. A smaller size electronic equipment, for example, a mobile computer, a potable telephone or an artificial satellite can be obtained by using the capacitor of the present invention, because many capacitor products are generally used in the equipment.